1. Field of the Invention
The present invention relates to a mask defect repairing method of repairing a defect on a photomask, particularly defects caused by foreign objects adhering to the photomask after pattern formation, as well as a related semiconductor device manufacturing method.
2. Description of the Related Art
In recent years, a photolithography step for a semiconductor manufacture process has had an increasingly significant challenge. An increasing reduction in the sizes of semiconductor devices has resulted in a growing demand for a reduction in the sizes of materials used in the photolithography step. Very high accuracy has been requested; the device design rule has already been reduced down to 45 nm and a pattern size to be controlled is at most 4 nm.
Very high accuracy is also requested for detection of defects in a photomask. Specifically, even defects of at most about 40 nm size must be able to be detected on a photomask.
What is called a defect repairing apparatus has been used to repair defects on photomasks. Specifically, conventional defect repairing techniques include evaporation of defects using laser beams which is described in Jpn. Pat. Appln. KOKAI Publication No. 2002-62637, etching of defects using focused ion beams (FIBs) or repair of defects by deposition which is described in Jpn. Pat. Appln. KOKAI Publication No. 4-237054, and etching of defects using a microprobe to which atomic force microscopy (AFM) is applied.
Defects that are difficult to repair have frequently resulted from a decrease in the sizes of defects and patterns on masks associated with the reduced sizes of semiconductor devices. That is, what is called adhesion defects has become difficult to repair. The adhesion defects have been removed by washing or repaired as described above.
However, in particular, patterns called sub-resolution assist features (SRAFs) are too fine to be resolved on a wafer. SRAF is formed on a wafer immediately adjacent to a pattern to be formed and has a size of at most 80 nm on a mask. Defects adhering between these fine patterns are difficult to repair using laser beams. This is because the laser beams have such a large diameter that they may damage not only the defects but also peripheral patterns.
The FIB repair is also difficult because a quartz substrate may be damaged by FIB specific repair marks or ions remaining into the substrate; the repair marks are called riverbeds. AFM enables repairs but requires a long time to complete the repair. This poses a throughput problem. EUVL masks may cut defects too deep. It is thus difficult to use these masks to repair the defects without damaging a base substrate.
In spite of these circumstances, the number of adhesion defects increases exponentially with decreasing pattern size. Defect repairs thus require a very long time. This is a major factor that increases mask costs.